1. Field of the Invention
The present invention relates to an angular velocity sensing device using a tuning-fork sensor or a three-forked tuning sensor comprising of a piezoelectric single crystal.
2. Description of the Related Art
An angular velocity sensing device using a mechanical rotor-based gyroscope has long been used as an inertial navigation system for aircraft and ships. The mechanical rotor-based gyroscope is excellent in stability and performance but, on the other hand, it has disadvantages owing to its large size, high cost, and short life.
In recent years, in place of the mechanical rotor-based gyroscope, development of small vibrating gyroscope for practical use has been proceeding, in which a vibration member is excited to vibrate with a polycrystalline piezoelectric device which is piezoceramic made of barium titanate and lead zirconate series, and voltage produced by a vibration caused by a Coriolis force induced by the rotational angular velocity is detected with a piezoelectric device.
For instance, an angular velocity sensing device comprised of a gyroscope using a tuning-fork sensor is proposed in Japanese laid-open patent publication No. Hei 3-10112.
Here, an angular velocity sensing device using a conventional gyroscope will be briefly explained. The angular velocity sensing device disclosed in the above-described publication has a configuration in which a piezoelectric device for driving is provided on a tuning-fork sensor having a central connecting section, and the central connecting section extending from a base of the tuning-fork sensor is supported by a cylindrical pipe member which serves as a case through a hinge section arranged in the direction orthogonal to the central connecting section.
The tuning-fork sensor is vibrated by applying AC voltage to the piezoelectric device for driving, and the hinge section performs bending vibration while deforming in an S-shape due to a Coriolis force induced by the rotation of the cylindrical pipe member. Voltage is produced in the piezoelectric device for detection provided on the hinge section and the angular velocity is obtained by detecting the produced voltage through a voltage detection circuit.
FIG. 26 shows the voltage detection circuit using the piezoelectric device for sensing the angular velocity.
In FIG. 26, the piezoelectric device 80 for detection can be expressed equivalently with a capacitor 86 (capacitance value C3), a voltage source 84 and a resistor 85 (resistance value R5). The piezoelectric device 80 is connected across a positive-input terminal of a operational amplifier 83 and a ground, and a resistor 87(resistance value R3) is connected across a negative-input terminal of the operational amplifier 83 and a ground, a resistor 88 (resistance value R4) is connected across the negative-input terminal and an output terminal to form the voltage detection circuit by an amplification circuit.
Accordingly, voltage Vi produced by the piezoelectric device 80 is amplified into V.sub.0 =(1+R4/R3) Vi through the voltage detection circuit to obtain output voltage V.sub.0 which can be treated. By synchronized detection of the output voltage V.sub.0 using a reference frequency of a tuning-fork sensor, the angular velocity can be obtained.
In a piezoelectric device comprised of a polycrystalline material such as piezoelectric ceramic used in the conventional example, the resistance value R5 of the equivalent resistor shown by the resistor 85 in FIG. 26 is a low value of less than 1 k.OMEGA. and has properties close to those of a constant voltage source as shown in the equivalent circuit.
Voltage produced on the piezoelectric device 80 by a piezoelectric distortion effect is in a range of several hundred microvolts to several millivolts, when angular velocity of one turn of the rotational movement of one degree per second is applied.
FIG. 27 is a graphic chart showing a theoretical limitation value of the voltage measurement and shows that noise voltage (V) linearly increases in proportion to source resistance that is an equivalent resistance (.OMEGA.) or impedance (.OMEGA.).
A noise voltage straight line 90 and a noise voltage straight line 91 are almost parallel, and a range above the noise voltage straight line 90 is the one where the produced voltage Vi can be easily detected by a simple amplifier, and a range above the noise voltage straight line 91 is the one where the produced voltage Vi can not be detected without precision measurement equipment such as an electrometer.
A range below the straight line 91 is the one where the produced voltage Vi can not be detected theoretically.
As described hereinbefore, a piezoelectric ceramic is used as a piezoelectric device, and source resistance of the piezoelectric ceramic that is an equivalent resistance or equivalent impedance is less than 1 k.OMEGA.. The noise voltage level of the noise voltage straight line 90 near the resistance value is about 1 microvolt. However, voltage produced by the piezoelectric ceramic is in a range of several hundred microvolts to several millivolts.
Accordingly, the angular velocity sensing device using a piezoelectric ceramic series piezoelectric device for detection can determine the angular velocity using a relatively simple voltage detection circuit as shown in FIG. 26.
However, such a conventional velocity sensing device has the following disadvantages.
First, since the tuning-fork sensor is configured to be supported with a cylindrical pipe member through a hinge which is provided orthogonally to the central connecting section extending from the base, the angular velocity sensing device is complicated in shape.
Further, since a plurality of piezoelectric devices need to be connected to a metal tuning-fork sensor and the hinge, the assembling process is complicated, the whole size becomes large, and cost reduction becomes difficult. Furthermore, since the metal tuning-fork sensor is used, characteristic of not being influenced by temperature is not satisfactory, and there arises a disadvantage that the characteristic is changed with time.
Second, if a single crystalline material having a source resistance (equivalent resistance) value or an equivalent electric impedance value of higher than 10 k.OMEGA. is used instead of the piezoelectric ceramic as a sensing device, voltage produced by a piezoelectric distortion effect is several microvolts in the case of an angular velocity of one degree per second.
As is clear from a chart of the noise voltage straight line 90 in FIG. 27, when source resistance (equivalent resistance) or equivalent electric impedance is more than 10 k.OMEGA., the noise voltage of the sensing device increases to more than 1 microvolt, and owing to the noise voltage, the angular velocity can not be detected with the conventional voltage sensor circuit shown in FIG. 26, which causes a disadvantage of difficulty in making the sensing device practicable.